The present invention relates to an oscillating circuit, and particularly, to an oscillating circuit to be used for a video signal receiving device having an improved VCO (voltage controlled oscillator) free run frequency characteristic, particularly, which serves to detect a video in a PLL (phase-locked loop) type video detecting circuit, for example.
In the PLL type video detecting circuit, a VCO circuit for locking to an input signal to detect a video is used. The VCO circuit regulates a free run frequency into an IF frequency (for example, 45.75 MHz specified for U.S.) to be used by means of an externally provided resonance circuit 51 formed by a variable coil L1 and a capacitor C1 as shown in FIG. 4. The resonance circuit 51 is connected to an oscillator 52 provided in an IC through an IC pin 54 of a lead frame (not shown) and a bonding pad 53 for connecting with an IC terminal. The resonance circuit may be constituted by an internal element (a capacitor C2 shown in a dotted line of FIG. 4) of an integrated circuit and the externally provided variable coil L1 in place of the externally provided capacitor C1.
In a VCO for video detection, a free run frequency is determined by the resonance circuit 51 and is locked to an input frequency through the PLL operation of an APC (automatic phase control) circuit. When an IC power is turned ON, an electric potential of an island provided under the bonding pad 53 is changed so that a floating capacity of the bonding pad is varied, resulting in a drift of the VCO free run frequency. In the case in which an AFT (automatic frequency tuning) voltage is to be generated by utilizing a change in the APC voltage with respect to the frequency, the drift appears as a drift of an AFT voltage. Since the AFT has a very high sensitivity, the drift causes a center voltage to be shifted.
For this reason, when a VCO coil is to be varied and regulated to generate a desirable AFT voltage, a VCO oscillating frequency cannot be regulated accurately or the regulation takes a great deal of time and labor if a drift is caused while the IC power is ON.
The cause will be described below with reference to a sectional view showing a bonding pad structure in FIG. 5. An IC bonding pad 63 is provided on an island formed in a P-type semiconductor substrate 61 which will be hereinafter referred to as an N-type epitaxial layer 62, for example. The epitaxial layer 62 provided under the pad is set in a floating state on an electric potential basis.
The bonding pad 63 is formed of a metal layer, a MOS capacitance Cm is formed between the epitaxial layer 62 and a single-layered metal 63A provided through an oxide film 64, and a junction capacitance Cj is formed between the (N-type) epitaxial layer 62 and the (P-type) substrate 61 (see FIGS. 5 to 7). Values of the parasitic capacities are changed depending on a difference in an electric potential between the epitaxial layer 62 and the substrate 61.
In the case in which the epitaxial layer 62 provided under the pad 63 is set in the floating state, differences in an electric potential determined by the values of the MOS capacitance Cm and the junction capacitance Cj are caused between the pad 63 and the epitaxial layer 62 and between the epitaxial layer 62 and the substrate 61 when a voltage is applied to the pad 63. Consequently, a very small leakage current (dark current) is generated from the epitaxial layer 62 to the substrate 61 of a chip. Therefore, a long time is taken until the electric potential of the epitaxial layer 62 is stabilized to be equal to that of the substrate 61. Therefore, the value of the parasitic capacity between the single-layered metal 63A and the epitaxial layer 62 is successively changed before the electric potential of the epitaxial layer 62 is stabilized to be equal to that of the substrate 61.
This change in the capacity influences the capacity of the resonance circuit 51 so that a frequency drift is generated when the power supply is turned ON.
In consideration of the problems, the invention is intended for fixing an electric potential of an epitaxial layer provided under a bonding pad to which a resonance circuit for a VCO is connected to a predetermined electric potential through a resistor, thereby increasing a speed of a change in the electric potential of the epitaxial layer and quickly stabilizing a value of a parasitic capacity, resulting in an improvement in a drift when a power supply is turned ON.
Moreover, an island provided under the bonding pad is fixed to have a predetermined electric potential through a resistor formed in another island.
Furthermore, the predetermined electric potential to which an electric potential of the island provided under the bonding pad is to be fixed is a Vcc voltage or a GND voltage.
Moreover, the bonding pad is formed in a position shifted from a position where a contact portion to be connected to the resistor is wire-bonded.